This is a copy of the notification I received from COX Cable in January 2017 (I was wrong on the overage charge, it applies to 50gb as opposed to 10gb) --
Dear (name removed),
We are writing to inform you of an upcoming change to Cox High Speed Internet service in your area related to data usage.
Your Cox High Speed Internet service currently includes a data plan of 1 TB (1,024 GB).
Beginning 02/20/2017, if you exceed your monthly data plan we will automatically provide additional blocks of data for $10 per 50 gigabytes (GB), as needed. This will not impact 98% percent of customers, but instead only charges the heaviest Internet users.
To help you get accustomed to this change, you will be provided a grace period for your first two billing cycles after the effective date. You will not be charged if you exceed your data plan during this grace period.
Your recent data usage history indicates you are unlikely to exceed your Cox High Speed Internet data plan of 1 TB per month.We will notify you by email and browser alert when you reach 85% and 100% of your monthly plan.
To view your data usage and learn more about data usage, visitcox.com/datausage. You can also view your data usage by downloading the Cox Connect mobile app.
Thank you for continuing to choose Cox as your Internet provider.